The present invention relates to a method for improving printing characteristics especially printing endurance of images of fine dots and fine lines formed on offset printing plate made using silver complex diffusion transfer process and furthermore inhibiting of occurrence of printing stain.
Lithographic printing plate consists of greasy ink receptive oleophilic image portion and ink repellent oleophobic non-image portions, the latter being generally water receptive hydrophilic areas.
Therefore, the customary lithographic printing is carried out by feeding both water and ink to the printing plate surface to allow the image portion to receive preferentially the colored ink and the non-image portion to receive preferentially water and then transferring the ink deposited on the image portion onto a substrate such as paper. In order to obtain a print of good quality, therefore, it is necessary that oleophilicity of the image portion and hydrophilicity of the non-image background portion are both strong enough so that when water and ink are applied the image portion may receive sufficient amount of ink while the non-image portion may completely repel the ink.
Generally, printing stain in lithographic printing does not occur due to simple causes, but occurs due to various causes. For example, the printing stain may be caused by properties inherent to printing plate materials, printing ink, dampening solution, printing paper, condition of printing machine, temperature and humidity at printing and others. Thus, it is important to properly arrange these printing conditions and environment. However, from practical viewpoint, there have been demanded materials which can be used for printing under conditions of as wide as possible.
This is the same for the materials fundamentally stated in Japanese Patent Kokoku (Post Exam. Publn.) Nos. 48-16725 and 48-30562, namely, these materials are narrow in latitude for commercial various inks, are apt to change in printing characteristics with time before and after making into printing plates, and are apt to show stain in long running of printing. Improvements of these defects have been desired.
Printing materials which suffer from less staining in printing were proposed in Japanese Patent Kokoku (Post Exam. Publn.) Nos. 56-24262 and 57-22376, but these materials still have the defects that receptivity of ink, printing endurance and image reproducibility are deteriorated during long use of them.